Reducing cement paste volume for production of SCC by adding fillers Professor Albert K.H. Kwan Department of Civil Engineering The University of Hong Kong Dr. Jaime S.K. Yeung Score Holdings Ltd.

Introduction The complex shape of some of today’s large scale infrastructure demand the uses of very large and sophisticated concrete moulds and exceedingly dense steel reinforcement, which together render concreting a formidable task. The great difficulties with the placing of concrete through closely spaced reinforcing bars into every corner of the mould and with the compaction of concrete placed inside confined space could lead to unfilled corners, honeycombing, insufficient steel-concrete bond and other defects. To improve the general quality of concrete construction, the use of self-consolidating concrete (SCC) is probably the best option.

Introduction SCC has excellent ability to deform and flow; fill up confined spaces and far-reaching corners; pass through small clearances between rebars; and achieve good consolidation without compaction (or with facilitation of very little compaction in some extremely difficult condition). Advantages of using SCC: enables a highly automated concreting operation that allows reduction in the number of concrete workers and improvement in site management.

Introduction Advantages of using SCC (continued): Without the need of vibration, the concreting speed can be accelerated. Without the need of vibration, the noise generated can be reduced by about 90% (8-10dB), leading to the possibility of extending the working hours to the evening and even night time. The automated concreting operation and longer working hours would together dramatically speed up the construction. The necessity to cast the concrete structure in stages can be eliminated and the provision of construction joints can be avoided.

Introduction It is not at all easy to produce SCC. In order for concrete to be classified as SCC, it should have the following properties: (1) High workability; (2) High passing ability; and (3) High segregation resistance. To achieve high passing ability and high segregation resistance, the concrete needs to have relatively high cohesiveness SCC must be designed to have high workability and high cohesiveness, which are not easy to achieve concurrently.

Introduction The addition of more superplasticizer to increase the workability would at the same time reduce the cohesiveness. Concrete producers are forced to increase the paste volume so as to achieve both high workability and high cohesiveness. In general a paste volume of 30% to 38% is needed. → quite large! What are the problems with concrete with large paste volume composed only of cementitious materials and water? (1) High material cost (2) Low dimensional stability (3) High hydration heat generated (4) High carbon footprint

Experimental Program Materials Cementitious Material: Ordinary Portland cement (OPC) of strength class 52.5N + locally produced pulverized fuel ash (PFA) Relative densities: OPC = 3.16; PFA = 2.52 Specific surface areas: OPC = 336 m 2 /kg; PFA = 369 m 2 /kg Fine Aggregates: Crushed granite rocks Nominal maximum size: 5 mm Relative density: 2.62

Experimental Program Materials Fine Aggregates (continued): Fineness modulus: 3.26 Water absorption: 0.8% Coarse Aggregates: Crushed granite rocks Nominal maximum size: 20 mm Relative density: 2.62 Fineness modulus: 6.46 Water absorption: 0.6%

Experimental Program Materials Superplasticiser (SP): Polycarboxylate-based Relative density: 1.05 Solid content: 20% Molecules have a comb-like structure consisting of a backbone chain and a number of graft chains

Experimental Program Materials Filler 1 – Limestone Fines (LF) Ground to have fineness similar to cement Volumetric mean particle size: 8.4  m Filler 2 – Ground Sand (GS) Ground to have maximum particle size of 600  m Volumetric mean particle size: 302  m It was expected that the LF would intermix with the cement paste to become powder paste with a larger volume whilst the GS would intermix with the mortar portion of the concrete to become part of the mortar

Experimental Program Mix Proportions (Percentage of Concrete Volume) Mix no. 1 LF content (%) GS content (%) Cement paste volume (%) Fine aggregate content (%) Coarse aggregate content (%) W/CM ratio A A A A A A Note: 1. Mixes are identified by the convention: (Series) – (LF content in %) – (GS content in %) – W/C ratio 2. SP was added until the slump flow reached at least 650 mm or the concrete mix was showing signs of segregation.

Experimental Program Mix Proportions (Percentage of Concrete Volume) Mix no. 1 LF content (%) GS content (%) Cement paste volume (%) Fine aggregate content (%) Coarse aggregate content (%) W/CM ratio B B B B B B Note: 1. Mixes are identified by the convention: (Series) – (LF content in %) – (GS content in %) – W/C ratio 2. SP was added until the slump flow reached at least 650 mm or the concrete mix was showing signs of segregation.

Experimental Program Mixing, Testing and Casting Experimental Procedures An electronic balance was used to weigh the materials and a pan mixer was employed to produce each batch of concrete. During production, the cementitious materials and water were first added into the mixer. After a while of preliminary mixing, the fillers, fine aggregate and coarse aggregate were added to the mixer. SP was then added bit by bit and the mixing was continued for about 10 minutes until the concrete mix appeared wet with paste formed.

Experimental Program Mixing, Testing and Casting Experimental Procedures (continued) Immediately after completion of the mixing process, concrete samples were taken from the mixer for slump flow test, L-box test and sieve segregation test, which were all performed within 30 minutes to avoid significant workability loss with time. After finishing these tests, the concrete samples were put back into the mixer for remixing and then taken out of the mixer for casting a total of nine 100 mm cubes. The cubes were cast on a vibration table. At 24 hours after casting, the cubes were demoulded and put into a lime-saturated water curing tank controlled at a temperature of 27 ± 2  C until the time of cube compression test.

Experimental Program Mixing, Testing and Casting Slump Flow Test The slump flow test for measuring the flowability, as stipulated in the European Guidelines for SCC. It is very similar to the slump test for conventional concrete stipulated in BS1881: Part 102: 1983 and the same apparatus were employed. Unlike the slump test, no tamping was applied when filling the concrete into the slump cone.

Slump flow tests for all the concrete mixes A A A A A A B B B B B B

Photos showing no segregation problem with all the concrete mixes A A A A A A B B B B B B

Experimental Program Mixing, Testing and Casting L-Box Test The L-box test for measuring the passing ability, as stipulated in the European Guidelines for SCC Apparatus: Note: All dimensions in mm gate 2 × 12 ϕ smooth bars with gap = 59 mm for PL1 3 × 12 ϕ smooth bars with gap = 41 mm for PL2 H1H1 ∆H 1 H2H2 L-box ratio = H 1 /H 2

L-Box Test A A A A A A B B B B B B

Experimental Program: Mixing, Testing and Casting Sieve Segregation Test The sieve segregation test for measuring the segregation resistance, as stipulated in the European Guidelines for SCC Apparatus and the test: Base receiver 5 mm sieve Electronic balance Sample container 500 mm 15 minutes

Sieve Segregation Test

Experimental Results Mix no. SP dosage (litre/m 3 ) Slump flow (mm) L-box ratio Segregated portion (%) 7-day cube strength (MPa) 28-day cube strength (MPa) Estimated adiabatic temperature rise (°C) A A A A A A B B B B B B

Discussions With the cement paste volume reduced from 35% to 25%, one immediate benefit is that the amount of cementitious materials to be added can be decreased by as much as 29%. Reduction of cementitious material by adding fillers Without the addition of fillers, a cementitious materials content of about 450 kg/m 3 is generally regarded as the minimum for the production of SCC. By adding limestone fines into the paste to increase the paste volume, the cementititous materials content has been reduced to 320 kg/m 3. By adding also ground sand into the mortar to increase the mortar volume, the cementitious materials content has been reduced to 300 kg/m 3.

Discussions Reduction of heat generation The substantial decrease in cementitious materials content due to the reduction in cement paste volume would significantly decrease the heat generation of the concrete during curing. The addition of fillers to reduce the cement paste volume to 25% can lower the adiabatic temperature rise by the order of 10 to 16°C at the W/CM ratios of 0.40 and This will largely reduce the need of costly temperature control for fresh concrete and the risk of thermal crack formation, especially in thick section pours. SCC mixes with cement paste volume reduced to 25% by the addition of fillers may be regarded as low-heat SCC and Green Concrete.

Discussions Theoretically, reduction of the cement paste volume would also decrease the shrinkage and creep, and increase the Young’s modulus of the concrete. In other words, the reduction of the cement paste volume down to 25% would significantly increase the dimensional stability of the SCC. Hence, the problem with the relatively low dimensional stability of SCC due to the large cement paste volume could be overcome by adding suitable fine fillers to reduce the cement paste volume.

Conclusions To study the feasibility of adding fillers to reduce the cement paste volume in SCC, an experimental program, in which two fillers, namely, limestone fines and ground sand, were employed for the production of SCC, has been conducted. The limestone fines, which has similar fineness as the cementitious material, was added to replace an equal volume of cement paste Whereas the ground sand, which has a mean particle size of 302  m, was added to replace one quarter of its volume of cement paste and three quarter of its volume of total aggregate.

Conclusions With up to 8% limestone fines and 8% ground sand added, the cement paste volume could be reduced to 25% while still satisfying the slump flow, passing ability and segregation resistance requirements of SCC. With the cement paste volume so reduced, the cementitious materials content could be decreased to lower the material cost, carbon footprint and temperature rise at early age. Moreover, the shrinkage and creep should be decreased and the Young’s modulus should be increased.

Conclusions The various problems associated with the large cement paste volume in SCC could be overcome by adding fillers to reduce the cement paste volume. SCC with fillers added to reduce the cement paste volume to only 25% should be regarded as second generation SCC.

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